1. Field of the Invention
The present invention relates to a piezoelectric sensor for detecting a sound wave (vibrations) which propagates in an object to be measured, and a coordinate input apparatus employing such a piezoelectric sensor.
2. Description of the Related Art
Coordinate input apparatuses are known, which are designed to detect sound waves (vibrations) which propagate in all directions from a position on a measuring plate at which a user brings a pen having a vibration element into contact with a measuring plate and to determine a contact position of the pen by detecting a transmission delay time of the propagation.
Such a coordinate input apparatus generally employs, as the vibration detection means for detecting vibrations, a piezoelectric ceramic, such as lead titanate zirconate (hereinafter referred to as PZT). The detecting element has a shape and a vibration mode which assure effective detection of a sound wave having a predetermined frequency and propagating in an object to be measured. In an actual element, the shape thereof is determined such that the mechanical resonance of the sensor coincides with the frequency of the sound wave to be detected, and the mode of the vibrations to be detected and so on are taken into consideration to determine the sensitivity of the element and the method of setting the element. In other words, the shape and so on of the vibration sensor are determined by the vibration mode or the frequency of the sound wave which propagates in the object to be measured.
Conventional piezoelectric elements will be described in more in detail below.
Vibrations of piezoelectric ceramics are classified into transverse effect vibrations in which the direction of an electric field (polarization) is perpendicular to the direction of deformation and longitudinal effect vibrations in which the direction of an electric field (polarization) is parallel to the direction of deformation. FIGS. 8(a) and 8(b) show vibration sensors which utilize transverse effect vibrations, and FIGS. 8(c) and 8(d) show vibration sensors which utilize longitudinal vibrations.
More specifically, the vibration sensor shown in FIG. 8(a) is called a radial vibrator which deforms in a radial direction. In this vibrator, a diameter d of the disk must be sufficiently large relative to a thickness t thereof. The vibration sensor shown in FIG. 8(b) produces vibrations of the bar and deforms in the directions of a length l. In this vibrator, the length l must be sufficiently large relative to a thickness t and a width w to assure free expansion and contraction of the bar in the direction of the length l. In both cases shown in FIGS. 8(a) and 8(b) , desirable values for l and d are at least ten times the thickness t.
The vibrator shown in FIG. 8(c) produces longitudinal vibrations of the bar and deforms in the direction of a length l. In this vibrator, although the length l must be sufficiently large relative to a width w to assure free expansion and contraction, a length l which is three to four times that of the width w is used in an actual vibrator because of the floating capacity or the like. The vibration sensor shown in FIG. 8(d) produces vibrations in the direction of a thickness thereof and deforms in the direction of a thickness t (the direction of polarization). In this case, the area of the vibrator must be sufficiently large compared to the thickness t.
As is clear from the foregoing description, the piezoelectric elements capable of detecting vibrations effectively have restrictions of their shape. This will be described in more detail with reference to FIG. 8(a). The element's resonance frequency associated with the piezoelectric transverse effect is determined by the radial length d of the element while the element's resonance frequency associated with the piezoelectric longitudinal effect is determined by the thickness t of the element. Therefore, if the resonance frequencies of both effects are close to each other, the vibration amplitude of the element is small due to vibration mode coupling, making it impossible to obtain a sufficient electric output. That is, as the radial length d and the thickness t become close to each other, the electric output signal level is reduced, and the function of the element deteriorates. The ratio of the radial length d to the thickness t must therefore be sufficiently large.
Various types of coordinate input apparatus utilizing the piezoelectric elements manufactured under the above-described restrictions have been proposed. Examples of such coordinate input apparatus include the one which employs a radial vibrator and the one which employs a columnar longitudinal vibrator.
Further, it is known that how a piezoelectric element is mounted on the vibration transmitting plate serving as an input surface affects the level of the detected signal. In an actual apparatus, the signal level when the piezoelectric element is set on the front surface of the vibration transmitting plate differs from the signal level when the piezoelectric element is set on the side surface of the vibration transmitting plate.
However, the conventional coordinate input apparatus has the following disadvantages.
When vibrations are detected using the above-described types of piezoelectric elements, direct electrical connection to the element cannot be obtained because the electrode surface of the element is adhered to the object to be measured.
Accordingly, it has been proposed to partially provide one of the electrodes of the piezoelectric element on the side surface thereof to obtain electrical conduction of the piezoelectric element from the side surface thereof using electrical conduction means. However, the provision of the side electrode greatly increases production cost in terms of the method and process of manufacturing the piezoelectric element from a piezoelectric ceramic. Further, in an element having small size, the provision of the side electrode reduces the distance between the side surface electrode and the other electrode, thus generating a problem involving insulation resistance (caused by application of a high voltage to the piezoelectric element in order to achieve polarization of the element). This makes manufacture of the element impossible.
Electrical connection to the piezoelectric element may also be obtained through an object to be measured if the object to be measured to which the piezoelectric element is adhered is conductive. However, in this case, the object to be measured must be made of a metal and a transparent member cannot be used to construct the object to be measured. In other words, it is impossible to provide a coordinate input apparatus in which an output device, such as a liquid crystal display, is disposed below the coordinate input surface so as to allow the user to input a coordinate as if he or she is drawing a picture on a sheet of paper with a pencil.
Where the coordinate input surface is constituted of a transparent plate-like member, such as a glass, printing an electrode pattern on the glass using a conductive ink or the like has been considered. However, this increases the number of manufacturing processes, thus increasing production cost. Moreover, the printed layer absorbs the sound wave, thus reducing the level of the detected signal (the entire efficiency).
The above-described methods of obtaining electrical connection to the element have the following disadvantages when they are applied to a coordinate input apparatus: whereas one of the electrodes connects through the object to be measured (including the electrode pattern formed on the object to be measured), the other electrode is formed on the surface of the element opposite to that on which the one electrode is formed. Accordingly, the provision of means for making that electrode electrically conductive (which may be a lead formed by soldering or pressing provided by a spring) increases the thickness of the apparatus. In this case, the thickness of the apparatus is the total of the thickness of the plate serving as the object to be measured, the thickness of the sensor and the space required to take out the electrodes.
If the above-described coordinate input apparatus is applied to a portable apparatus (a pen input computer or the like) which has been drawing attention in recent years, it places restrictions of the thickness of the apparatus and adversely affects design thereof.